Fluid flow systems, such as natural gas distribution systems, include a network of fluid flow lines that distribute fluid from a fluid source, such as a gas provider, to a number of individual users at, for example, houses, apartment buildings, etc. Typically, the fluid flow lines associated with a gas distribution system are disposed underground and have regulators, valves and/or other fluid flow control components disposed at advantageous control points within the lines. The control components regulate the flow of gas from a high-pressure side associated with the fluid provider to a low-pressure side associated with a fluid user.
To aid in the maintenance of, for example, gas lines, the regulators, valves and other components at a control point are disposed in above-ground stations or pit assemblies that allow access to these components. Pit assemblies are usually located underground and are large enough so that a service person can enter the pit assembly to access, test, repair and/or replace the regulators or other flow control components within the pit assembly.
In a standard configuration, each pit assembly includes two sets of redundant regulators connected in parallel between a gas inlet (coupled to a gas provider) and a gas outlet (coupled to a gas user). One of these sets of regulators comprises a main flow line that is used to control fluid flow during normal operation of the gas system while the other set of regulators comprises a backup line that remains unused while the main line is operational. In another standard configuration, two pit assemblies are placed in series separated by a length of pipe, per local requirements, with one set of regulators in each pit assembly. During operation, one of these sets of regulators may be used to actively control fluid flow during normal operation of the gas system while the other set of regulators may comprise a backup line that monitors flow when the active set is operational (a worker/monitor set-up). Alternatively, both of these sets of regulators may be used to control fluid flow during normal operation so as to maintain an intermediate pressure between the pit assemblies (a series regulator set-up). In either of these cases, by-pass lines would be included to allow isolation of the regulator sets from the main line.
When one or more of the regulators or other components associated with the main line needs to be tested, repaired or replaced, the backup or by-pass line is switched into use to provide a constant flow of gas to the gas users. Thereafter the main line is disassembled and the components thereof are tested, repaired or replaced as necessary. This disassembly procedure can take a significant amount of time due to the number of nuts, bolts, and other fasteners which must be removed to disassemble the components of the main line. During this time, surface traffic flow around the installation may be disrupted. Furthermore, removal of components of either the main line or the backup line is impaired by the limited amount of space within the pit assembly and by the hard-to-reach nature of some of the components within the pit assembly. Sometimes, special precautions must also be taken before entering a pit assembly to perform test, repair, or maintenance work. Such precautions may include venting the pit assembly, which requires an external air supply and adds to the time and cost associated with performing maintenance. It is desirable to reduce the size of a pit assembly and to make fluid flow lines less expensive to build and faster, safer and easier to maintain.